In LTE (Long Term Evolution)/LTE-A (Advanced) which is a next-generation mobile communication system, with the aim of expanding system capacity and coverage, a heterogeneous network in which macrocells and picocells (including femtocells) coexist has been considered. In this network, these cells are operated at the same frequency; therefore, interference generated between cells becomes a problem. For example, in a case where macrocells and picocells are intermingled and operated, in a mobile station connected to a low-transmission-power pico base station, a downlink desired signal from the pico base station suffers significant interference from a downlink of a macro base station, and the communication quality becomes degraded.
As a technology for reducing such intercell interference, there is FFR (Fractional Frequency Reuse). In the FFR, a pico base station assigns a specific frequency resource to a downlink shared channel to a mobile station on the cell boundary. On the other hand, a macro base station does not transmit a downlink shared channel on the frequency resource, or, if transmitting a downlink shared channel on the frequency resource, transmits the downlink shared channel at low transmission power. Consequently, it is possible to reduce intercell interference in a downlink shared channel.
Meanwhile, downlink control channels are distributed over the entire system bandwidth; therefore, it is difficult to apply the FFR. Therefore, in a system where the timing to transmit a subframe is synchronized between cells, intercell interference occurs between downlink control channels. Thus, conventionally, the timing control between cells is made so that downlink control channels do not overlap, thereby reducing intercell interference in the downlink control channels. Specifically, a macro base station shifts the transmission timing of a pico base station in units of an OFDM symbol so that downlink control channels do not overlap between cells. Then, the macro base station overrides (mutes) a downlink shared channel of the macro base station, which overlaps with a shifted downlink control channel of the pico base station, with a null symbol for zero transmission power. By performing this operation, interference from a downlink shared channel of the macrocell to a downlink control channel of the picocell can be reduced.    Non Patent Document 1: 3GPP TR 36.814 V9.0.0 (2010-03)    Non Patent Document 2: 3GPP R1-103227 (2010-05)
However, in the above-described “method to reduce intercell interference in downlink control channels”, the downlink shared channel of the macrocell is overridden with the null symbol for zero transmission power; therefore, in the side of a mobile station, a reception characteristic of the downlink shared channel of the macrocell becomes degraded. Especially, when a downlink radio channel state of the macrocell is good, the number of code blocks, which are units of coding, in one subframe is increased, so if a whole code block is overridden with a null symbol, it is not possible to decode the code block. Namely, in such a case, there is a problem that it is not possible to restore original information bits.